Abstract (All changes from the previous submission are marked by line in the margins) Ischemia and reperfusion (I/R) injury results from an acute increase in oxidative/inflammatory stress during reperfusion after ischemia and triggers a cascade of pathophysiological events. The injury culminates in the death of cardiomyocytes that were viable immediately before myocardial reperfusion and occurs despite timely reperfusion and can increase infarct size (1). Currently, there is no drug that is utilized in the clinical arena that prevents or attenuates I/R injury in the patient presenting with acute coronary syndrome (1,2). I/R results in cardiomyocyte death and is associated with nuclear degeneration and myofibrillar degradation. Previous studies showed a significant increase of interstitial matrix metalloproteinase-2 (MMP-2) and MMP-9 activation after I/R (3). Recent studies demonstrated activated MMP-2 within the cardiomyocyte that cleaved myofibrils (4-6) and nuclear matrix protein poly ADP-ribose polymerase (PARP)-a DNA repair enzyme in cell survival during I/R (7). However, the regulation of intracellular MMPs during I/R is not known. Here, in a dog I/R model, we report that interstitial mast cell chymase plays an important role in cardiomyocyte MMP activation. Degranulation of resident mast cells is an early event in I/R and results in chymase release into the interstitium (8- 10). In addition to its ability to convert angiotensin (Ang I) to Ang II (11,12), mast cell chymase activates interstitial MMPs (13-18) and directly degrades cell surface proteins such as fibronectin (19), resulting in smooth muscle cell (20,21) and cardiomyocyte (22) death. In this proposal, we present preliminary data that increased interstitial chymase proteolytic activity after I/R in the dog in vivo results in a robust activation of intracellular MMPs within the cardiomyocyte nucleus along with DNA damage and loss of myosin. MMP activity was significantly attenuated by pretreatment with an orally active chymase inhibitor. We also found that I/R increased activity of two highly abundant unknown MMPs with molecular weight range 100 - 150 kD in the cardiomyocyte nucleus. Further, we found that chymase added to adult dog cardiomyocytes (plated on laminin) resulted in direct cleavage of laminin, focal adhesion kinase (FAK) dephopsphorylation, MMP release and myosin degradation. Chymase-induced myosin degradation was prevented by a broad spectrum MMP inhibitor and EDTA. Based on our preliminary in vivo and in vitro data, we hypothesize that increased ISF chymase during I/R mediates loss of cell-matrix-surface connections, disrupting FAK and culminating in myofibrillar degeneration through nuclear MMP activation. This hypothesis will be tested in a clinically relevant dog model of I/R by intravenous infusion (IV) of chymase inhibitor started 30 min after ischemia and continued throughout I/R with evaluation of LV function acutely and after chronic oral chymase inhibitor. Objective 1. Determine whether I/R results in activation of novel cardiomyocyte nuclear MMPs using proteomics and mass spectrometry approaches. Identification of novel nuclear MMPs and their regulation will provide a potential new molecular/protein target in I/R injury. Nuclear protein extraction from I/R cardiomyocytes will be separated by isoelectric focusing (IEF). After IEF, proteins on agarose gels will be separated using 2D zymography. Protein spots on the 2D gel corresponding to the areas of enzyme activity will be selected for identification of the MMP by mass spectrometry. Objective 2. Test the hypothesis that increase in ISF chymase activity during reperfusion leads to inactivation of FAK and downstream signaling that activate cardiomyocyte nuclear MMPs. Serial biopsy samples will be taken from the ischemic and nonischemic areas during I/R in dogs. We will study the time course of FAK dephosphorylation and activation of downstream signaling pathways ERK, JNK, p38 MAP kinase and NFB, which are known to activate MMPs. To address this question in a clinically relevant fashion, serial biopsy samples will be taken before and after reperfusion with and without IV infusion of chymase inhibitor started at 30 min of ischemia and throughout the reperfusion. LV diastolic and systolic function will be assessed using the LV impedance catheter during I/R. In situ zymography with immunohistochemistry will define whether nuclear MMP activation can be prevented by prior IV chymase inhibitor infusion. Objective 3. Test the hypothesis that intravenous chymase inhibitor infusion during I/R followed by oral chymase inhibitor treatment for 7 days results in reduced injury and improved LV function. In our closed chest animal model, the proximal left anterior will be occluded using percutaneous coronary intervention balloon occlusion for one hour and reperfusion for two hours. Intravenous chymase inhibitor will be started after 30 minutes of ischemia and continued throughout the two hours of reperfusion. Oral drug will be initiated within 12 hours and continued for 7 days. Magnetic resonance imaging (MRI) with tissue tagging and gadolinium perfusion will be performed at 2 days and 7 days after I/R injury. T2 weighted MRI at 2 days will determine area of edema in vivo, while gadolinium plus tissue tagging at 7 days will determine in vivo myocardial scar and function in vehicle vs. chymase inhibitor treated dogs.